Devices for carrying out thermoelectric cooling, thermoelectric heating and thermoelectric power generation using the thermoelectric properties of a thermoelectric semiconductor generally have a basic configuration where a plurality of thermoelectric modules 1 are aligned and connected in series, as shown schematically in the example of FIG. 27. In each of the thermoelectric modules 1, a PN element pair is formed by joining a P type thermoelectric semiconductor element 2 to an N type thermoelectric semiconductor element 3 via a metal electrode 4.
One type of thermoelectric semiconductor that forms above described thermoelectric semiconductor elements 2 and 3 uses a complex compound made of one or two elements selected from bismuth (Bi) and antimony (Sb) of 5B group, and one or two elements selected from tellurium (Te) and selenium (Se) of 6B group. The thermoelectric semiconductor is made of an alloy having a (Bi—Sb)2(Te—Se)3 based composition in which the ratio of a number of atoms of 5B group elements (Bi and Sb) to a number of atoms of 6B group elements (Te and Se) is 2:3.
Above-described alloy having a (Bi—Sb)2(Te—Se)3 based composition for forming the thermoelectric semiconductor, has a hexagonal structure, and electrical and thermal anisotropy due to the crystal structure. It is known that by conveying electricity or heat in the <110> direction of the crystal structure, that is, along C face of the hexagonal structure, excellent thermoelectric performance can be obtained, in comparison with a case where electricity or heat is conveyed in the direction of c-axis.
Conventionally, raw alloys prepared so as to have the above-described desired composition are heated and melted to form molten alloys. Subsequently, using a directional solidification method, such as a zone melting method, while controlling the direction of the crystal growth so that the crystal has an excellent thermoelectric performance along the growth direction, a single crystalline or a polycrystalline ingot is manufactured as a thermoelectric semiconductor material. By a required working of the ingot, such as cutting a portion having little irregularity in the composition from the ingot and working the cut portion, an element having an excellent properties is manufactured.
However, the ingots converted to single crystal using the zone melting method have significant cleavage due to their crystal structure. Therefore, when a thermoelectric semiconductor element is manufactured by slicing or the like of the ingot as a thermoelectric semiconductor material, there is a problem that the insufficient mechanical strength cause a reduction of yields by cracking or chipping. Therefore, it has been desired to improve thermoelectric performance along with increasing the strength of thermoelectric semiconductor materials for thermoelectric semiconductor elements.
In order to improve the strength and thermoelectric performance of thermoelectric semiconductors, one technique is proposed in which an ingot as a thermoelectric semiconductor material which has been manufactured in the same manner as described above by a directional solidification method, is worked by extrusion or rolling so as to apply shear force in the direction of C face of a hexagonal structure, and thereby improving the strength of the material (see, for example, Patent Document 1).
There has been proposed several method in view of general properties of polycrystalline metallic material as following: Crystal grains of polycrystalline metallic material show dispersive distribution of orientation, the metallic material exhibits isotropy. When the crystal grains are oriented in a specific direction as a result of a working such as plastic working, crystal anisotropy of individual crystal grains appears as macroscopic characteristics so that the metallic material as a whole exhibits anisotropy (for example, Non-Patent Document 1). By crushing raw alloy powder and sintering the powder, mechanical property of the material is improved in the sintered body. In the sintered body crystalline orientation is reduced, since the integration of randomly oriented powder grains during the sintering process orientates constituent crystals randomly. By rolling the sintered body in a direction (see, for example, Patent Document 2), by extrusion molding the sintered body (see, for example, Patent Documents 3 and 4), or by plastically deforming the sintered body (see, for example Patent Documents 5, 6, 7, 8, 9 and 10), uniformity of crystalline orientation of the sintered body is improved.
That is to say, by applying a pressing force on the above-described sintered body, and plastically deforming the sintered body, constituent crystals of the texture are plastically deformed and flattened in a direction perpendicular to the direction of pressing force, and thus, the crystals are oriented in such a manner that the cleavage plane are perpendicular to the direction of compression. In a rolling or a forging by an uniaxial compression, C face of the hexagonal structure is oriented in the direction perpendicular to the direction of compressing the sintered body (direction of pressing). In an extrusion molding, C face of the hexagonal structure is oriented along the direction of extrusion (direction of pressing). By this method, it is possible to prepare a thermoelectric semiconductor material in which crystals are oriented in a direction of excellent thermoelectric performance.
In general, the thermoelectric performance of the material used for the manufacture of a thermoelectric semiconductor is expressed by the following equation:Z=α2σ/κ=α2/(σ·κ)where Z is a Figure-of-Merit, α is the Seebeck coefficient, σ is electric conductivity, κ is thermal conductivity, and ρ is resistivity.
Accordingly, in order to increase the thermoelectric performance (Figure-of-Merit Z) of a thermoelectric semiconductor material, a raw alloy material in which the value of the Seebeck coefficient (π) or the electric conductivity (σ) is increased, or the thermal conductivity (κ) is lowered, may be utilized.
Judging from this, it should be possible to increase thermoelectric performance (Figure-of-Merit Z) by decreasing the grain sizes of crystals and reducing the conductivity (κ). However, in the above-described techniques using a powder produced by crushing an ingot of the raw alloy, the particle sizes of the powder is the grain sizes of crystals, therefore there is a limit to the miniaturization of crystal grains formed by crushing.
Therefore, in order to improve the strength and thermoelectric performance of a thermoelectric semiconductor material, still another technique has been proposed. A raw alloy is melted into a molten alloy. A raw thermoelectric semiconductor material in a ribbon, foil piece or powder form is formed by a liquid quenching method such as rotational roll method in which the molten alloy is sprayed onto the surface of a rotational roll which is being rotated or a gas atomizing method in which the molten alloy is sprayed into a predetermined gas flow. At that time, microscopic crystal grains are formed within the texture of the raw thermoelectric semiconductor material, and high density strain and defects are introduced into the texture. After the raw thermoelectric semiconductor material is crushed into a powder, this raw thermoelectric semiconductor material in powder form is heat treated and solidified, and thereby a thermoelectric semiconductor material is manufactured. By this method, during the heat treatment or the solidification process, recrystallization of crystals occurs using the distortion due to the defects as a driving force, and due to the presence of grain boundaries, the thermal conductivity (κ) is lowered and thermoelectric performance (Figure-of-Merit Z) is increased (see, for example, Patent Document 11).
As the rotational velocity of a rotational roll that is used to form a raw thermoelectric semiconductor material in a ribbon, foil piece or powder form by quenching a molten alloy, it is proposed to set a circumferential velocity to be 2 to 80 m/sec, so as to effectively generate microscopic crystals by quenching, and make the crystals grow in the direction of heat flow (see, for example, Patent Document 12). In this case, a sufficient quenching speed is not achieved when the circumferential velocity of the rotational roll is less than 2 m/sec, and a sufficient quenching speed is also not achieved when the circumferential velocity is 80 m/sec or greater.
As the heating conditions when the raw thermoelectric semiconductor material in a ribbon, foil or powder form is solidified and formed, it is proposed to maintain the material at a temperature from 200 to 400° C. or at a temperature from 400 to 600° C. for 5 to 150 minutes while applying pressure to the material (see, for example, Patent Document 13).
Another technique for increasing the thermoelectric performance of a thermoelectric semiconductor material is proposed, in which Ag is added to and mixed with a raw thermoelectric semiconductor material in a ribbon, foil piece or powder form that has been formed by quenching a molten alloy of a (Bi—Sb)2(Te—Se)3 based raw alloy, on a rotational roll. By subsequent sintering and solidification, Ag is distributed in the grain boundaries, so that resistivity ρ is lowered, and thus, an increase in the thermoelectric performance (Figure-of-Merit Z) can be achieved (see, for example, Patent Document 14).
It is known that in a rotational rolling method as the liquid quenching method, a molten alloy sprayed onto the surface of a rotational roll is cooled from contact surface with the rotational roll in the direction toward the outer periphery of the roll. Together with this quenching, the molten alloy solidifies in the direction of the film thickness. As a result, a raw thermoelectric semiconductor material in foil form is produced, in which C face, the base plane of the hexagonal structure of the crystal grains, stand in the direction of the film thickness.
Therefore, a technique for effectively using the orientation of the crystals of a raw thermoelectric semiconductor material that has been manufactured by the rotational rolling method is proposed, in which the raw thermoelectric semiconductor materials are layered in the direction of the film thickness, and are sintered while pressure is applied in the direction parallel to the direction of the film thickness, and thereby, a thermoelectric semiconductor material is manufactured (see, for example, Patent Document 15).
Furthermore, techniques for manufacturing a thermoelectric semiconductor material in which crystal orientation is improved have been proposed. In a technique, a layered body is produced by layering raw thermoelectric semiconductor materials manufactured by a rotational rolling method, and integrating the layered body layered in the direction of the film thickness by applying a pressure in the direction parallel to the layering direction. During the pressing for integrating the layers in the direction parallel to the layering direction, crystal orientation of each layers are disordered at the interface of the layers. By applying pressure in the direction perpendicular to the layering direction of the layered body, such disorder of crystal orientation at the interface can be improved (see, for example, Patent Document 16). In another technique, a layered body is produced by layering raw thermoelectric semiconductor materials in foil powder form in the direction of the film thickness. Crystalline orientation of the layered body is improved by applying pressure in at least three directions perpendicular to the layering direction. Furthermore, the layered body, the crystalline orientation of which has been improved by the above-described application of pressure, is formed by extrusion molding in the direction parallel to the layering direction, and thereby uniformity in the orientation of the crystals is additionally increased (see, for example, Patent Document 17).
Recently, it has been desired for a thermoelectric transducing material to be provided with further improved performance and high reliability. Together with an increase in performance, an increase in mechanical strength and excellence in workability are also desired. For example, when a thermoelectric semiconductor is used to cool a laser oscillator, N type and P type thermoelectric semiconductor elements having dimensions of no greater than 1 mm are used as modules. Accordingly, it is required a mechanical strength sufficient to make it possible for a thermoelectric semiconductor element of no greater than 1 mm in dimension to be sliced from an ingot of a thermoelectric semiconductor material without chipping.